In current semiconductor industry, integrated circuits can be classified into three types including logic circuits, memories and analog circuits. Memory devices, such as Random-Access Memory (RAM), Dynamic Random Access Memory (DRAM), Read-Only Memory (ROM), Erasable Programmable Read-Only Memory (EPROM), Flash Memory or Ferromagnetic Random Access Memory (FRAM), occupy a relatively large portion of integrated circuit products. Particularly, flash memories have been developed rapidly and have multiple advantages. For example, flash memories can store information permanently without being provided with power, have a high integration level and high access speed, and is easily erased. Therefore, flash memories are widely used in multiple fields, such as microcomputer or automation control.
Flash memories can be classified into two types, including stacked-gate devices and split-gate devices, where a stacked-gate device further includes a floating gate and a control gate on the floating gate. It is easier to manufacture a stacked-gate device than a split-gate device. However, stacked-gate devices generally have over-erase problem. To solve the problem, verification needs to be performed after erasure circulation to maintain a threshold voltage of a cell within a voltage range, which may increase the complexity of circuit design. To a split-gate device, a control gate thereof serves as a select transistor, which may effectively avoid over-erase effect and simplifies circuit design. Besides, in the split-gate device, programming is performed by hot electron injection at source, which may have higher programming efficiency. Therefore, split-gate devices are widely used in various electronic products, such as smart cards, Subscriber Identity Module (SIM) cards, microcontrollers or mobile phones.
More information about split-gate flash memories can be found in Chinese patent Publication No. CN103050446A (publication date: Dec. 20, 2012).
Due to poor erasure ability, split-gate flash memories formed by existing methods have poor reliability and durability.